Picking a Wireless Standard

Is There Currently a Leading Standard for Connecting Distributed I/O and Sensors That's a Safe Pick for the Future?

By Control Design Staff

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Our printing machines have widely distributed I/O and sensors, and we could save money by connecting some of these points via a wireless network. We hesitate to proceed because we're not sure where the current wireless standards are headed, and we don't want to implement a supposed standard that will fall out of favor in a few years. Is there currently a leading standard that would be a safer pick for the future?

—From October '12 Control Design


Standards Aren't the Thing
Wireless can indeed lower costs and even improve reliability in some cases (for example, eliminating possible cable failures due to harsh environments or maintenance issues encountered with slip rings/festooning on moving machines). But the savings are only achieved if the appropriate wireless technology is chosen and the wireless network is designed correctly. Reliability and compatibility are often more important than complying with a standard.

The first step is to review the current control system architecture — is it possible to adapt parts to wireless? The distributed I/O might be using some type of fieldbus, and a few (such as Ethernet) are adaptable to wireless. Sensor interfaces also vary, from simple 4–20 mA loops, to more sophisticated fieldbuses. It might be necessary to redesign the automation network to better accommodate wireless.

From a wireless I/O standard perspective, there are two dominant standards today: WirelessHART (IEC 62591) and ISA100.11a. Both have multi-vendor support and promise to have good market longevity. However, both standards are focused primarily on wireless field devices (instruments) for the process control markets. These networks are field-proven to be highly reliable and robust, but they are based on RF frequency-hopping techniques (IEEE 802.15.4) that adapt well in noisy environments but may have too much jitter (delay) and too slow scan rates for applications in discrete processes.

I suspect that your printing machine distributed I/O and sensors require faster scan rates than what WirelessHART or ISA100.11a support. If the system is PLC-based, easy PLC integration may be more important than following a wireless I/O standard. If so, a Wi-Fi-based solution (such as 802.11n) might be the best choice. The 802.11n standard offers very high-speed data connections for Ethernet, and most PLC vendors support Ethernet (EtherNet/IP, Profinet and Modbus TCP/IP being the most popular). Using wireless Ethernet (Wi-Fi) then provides a high-speed, low-latency connection to Ethernet-based distributed I/O blocks and sensors. These types of wireless solutions have been used in manufacturing and by machine builders for many years.

The best approach is to determine which technology is the best fit for your application. Standards come and go, but fortunately industrial communication vendors offer products with long lifecycles. Evaluate the company as well as the technology/products. How many years have they been in business? How much industrial wireless experience do they have? How knowledgeable about your application are their support engineers? Wireless can indeed lower costs, but choosing the best technology for your application is the key to success.

Jim Ralston,
strategic wireless manager,
ProSoft Technology

Explore 802.11n
Connecting I/O to sensors and actuators through a wireless network can save money, but you need to examine these I/O points and determine whether any of these points are deterministic and/or mission-critical. For example, a wastewater treatment facility could use a wireless sensor to indicate that a tank has reached capacity. Whether the PLC then communicates wirelessly, instructing the valve to turn off the flow in 0.01 s or 1 s is not relevant. A printing press, bottling plant or motion-oriented application might not have that degree of freedom.

If you feel comfortable with the wireless response time you can get in your printing machine application, you will want to explore Ethernet 802.11n. It is the leading standard for wireless Ethernet, and it offers improvement over susceptibility to interference from a broad range of wireless devices and industrial infrastructure. The 802.11n was designed with the goal of reaching 100+ Mbps, but is now projected to be reliable at 200+ Mbps. In an industrial setting, this gives the user quite a bit of room to grow.

The 802.11n standard offers the robustness, throughput, security and quality of service (QoS) capabilities that plant managers have come to expect from wired Ethernet networks. Part of this robustness is an added stream of data — 802.11n uses four beams instead of three. It also allows wireless transmissions to be shaped to arrive in phase when they get to the receiver. All other signals that arrive already out of phase are then cancelled out, cutting down interference and further improving reliability.

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